A GKNERALIZED YIl-LD TABLE 383 



Tliis means that by plottinj^ data for the best and poorest sites and 

 drawing maximum and minimum curves, as many interior curves with 

 even intervals can then be drawn as may be desired,, ahhough some 

 interior data, showing height growth of individual trees, are highly 

 desirable to establish and support the proper direction of all the curves. 



A central point in future age-height site classification work, it seems 

 to me. will be to compare and co-ordinate the age-height classes of 

 different species. This can not conveniently be done on the basis of 

 site class curves for the different species, as the direction of their height 

 growth curves varies too much at dift'erent periods in their lives. It 

 becomes necessary, therefore, to choose a single age at which to com- 

 pare the height classes attained by different species — a point Professor 

 Roth has been stressing for the last five years. Roth's recommenda- 

 tion of 100 years seems to me to be a suitable age to choose, as it 

 would occur shortly after the culmination of the main height growth 

 in all but a few rapid growing weed species. To make comparisons 

 between species most readily, it would obviously seem best to adopt a 

 standard series of 100-year height classes with sufficient range to cover 

 all species in the country and with a sufficient number of intervals and 

 classes so that age-height site curves for any given species can be made 

 to coincide with \hese standard intervals at 100 years without serious 

 distortion of supporting data. By drawing a set of curves for a given 

 species through such a set of standard 100-year-height intervals, it 

 should be possible to at once precisely co-ordinate its growth wath 

 that of other species. Standard 100-year-height classes would also 

 facilitate correlation and comparison, in terms of different species, of 

 the productive capacitv of dift'erent physical types or complexes of 

 physical factors. A piece of land constituting a physical type of a given 

 description may often be in one standard height class in terms of one 

 species and in another class in terms of another species. 



The follow-ing is suggestive of some possible standard height classes ^ 

 and methods of numbe^ring them (from the ground up) which it would 

 be useful to consider in connection with the subject of standard 100- 

 year height classes, using 10-foot and 20-foot intervals and with a 

 range in height from to 210 feet: 



' These classes are on the basis of a single standard for all species in contrast 

 with the triple standard proposed by Roth ; his standard "A" would be for 

 species attaining 150 feet or more, "B" for those attaining 120, and "C" for 

 those only reaching 90 feet ; for each one of which standards the sites would 

 be numbered independently. 



